The present invention relates to quality alarm systems and in particular to quality alarm systems for electric discharge lasers used for integrated circuit lithography.
An important use of gas discharge ultraviolet lasers such as KrF excimer lasers is as light sources for integrated circuit fabrication. These lasers are typically used with lithography machines called stepper and scanner machines. Lithography lasers are typically operated in a xe2x80x9cburstxe2x80x9d mode in which a large number (e.g. 85) die sections of a wafer are illuminated one-at-a-time. A typical burst of laser pulses (such as about 400) at (for example) 10 mJ per pulse is delivered at a repetition rate of about 2000 pulses per second to illuminate a single die section in about 0.2 second. After a die section is illuminated the laser is idle for a very short period (such as about 0.3 second) while the lithography equipment positions the wafer and the lens equipment of the lithography machine to illuminate the next die section. This on-off process continues until all die sections on the wafer have been illuminated at which time the laser is idle for a few seconds (such as about 8 seconds) while a new wafer is loaded for processing. This cycle continues as a part of an integrated circuit fabrication line which operates 24 hours per day, 7 days per week. All equipment must be extremely reliable because any unscheduled downtime can cost many thousands of dollars in lost production. In addition, it is extremely important for high quality circuit fabrication that the quality of the laser beam from the laser light source be maintained within narrow quality limits.
What is needed is a laser having intelligent internal monitoring, and preselectable quality standards and a notification technique to alert laser users as to when the beam quality is adequate for production and when it is not.
The present invention provides an electric discharge laser with an associated programmable controller which (1) will permit an operator to specify beam quality parameters based on historical data (2) will monitor those parameters and (3) will provide a notification signal to the operator informing him when the beam quality is adequate for integrated circuit fabrication. The controller is programmed to indicate an out-of-control condition when one or more quality parameters exhibit certain specified non-random behavior such as two out of three quality measurements deviating by more than 4 standard deviations and/or three out of four quality measurements deviating by more three standard deviations. In a preferred embodiment the program is designed to detect poor quality by looking for these xe2x80x9crunsxe2x80x9d of bad quantity data and to produce false indication of system out of control, on the average, no more than once each two months.